Acid neutralising capacity of two different bauxite residues (red mud) and their potential applications for treating acid sulfate water and soils

The acid neutralising capacity (ANC) and potential beneficial uses of 2 different bauxite residues (red mud) were investigated. The results show that the ANC is much higher in the red mud disposed of using a dry stacking method at the Pingguo Alumina Refinery (China) than in the red mud disposed of by a wet method using seawater at the Queensland Alumina Ltd Refinery (Australia). The higher ANC in the Pingguo red mud is attributable to its high CaO and low SiO2. An incubation experiment showed that leaching of alkaline materials from the lime-treated sample was much greater than that from the red mud-treated sample. This suggests that red mud may be superior to lime for treating potential acid sulfate soils, which contain sulfide minerals that could take a long time to oxidise and release soluble acid. The effects of 2 acid-filtering systems were tested, both of which used red mud as the main material for removal of acid from passing acidic water. The results showed that the red mud–CaCO3 filter performed better than the red mud–Mg(OH)2 filter. Results from pot trials in Australia further demonstrated that the application of combined red mud and sewage sludge significantly improved the soil conditions for the growth of 5 Australian native tree species, in addition to Eucalyptus paniculata, which successfully grew in the same mine soil amended with the red mud and sewage sludge in previous work of G. Maddocks et al. The results from the pot experiment in China showed that the application of combined neutralising agents (red mud/lime blends) and sewage sludge to the extremely acidic mine soil was insufficient for creating appropriate ecological conditions for the growth of vetiver grass. In this experiment, additional application of zeolitic rock powder significantly improved the growth performance of the plant.